Casting epoxy compositions are widely used as electrical insulating materials (for example, insulators) due to balanced mechanical and electrical properties, heat and chemical resistance of epoxy resins, compared to incumbent liquid silicone rubber based insulators. However, casting epoxy compositions still have some issues, especially in regions having high levels of precipitation and air pollution. In these regions, a conductive dirt and/or water layer may form on the surface of the insulator, which can lead to current leakage, arcing, and may result in insulator damage or even total failure. To avoid damaging water and water-born contamination on an epoxy composition insulator material, it is desirably that the epoxy compositions have a hydrophobic surface.
To increase hydrophobic surface properties of epoxy compositions, polysiloxanes (for example, hydroxyl-terminated polysiloxanes) owing to their low surface energy, are usually added into epoxy resins. However, since polysiloxanes and epoxy resins are thermodynamically immiscible, simply mixing them usually will not provide satisfactory hydrophobic surface properties. Satisfactory hydrophobic surface properties refer to a surface having a contact angle with deionized water that is 90° or greater.
To increase compatibility between epoxy resins and polysiloxanes, one incumbent solution is to provide a composition comprising a cycloaliphatic epoxy resin, a hydroxyl-terminated (OH-terminated) polysiloxane, a cyclic polysiloxane, and a non-ionic and fluoroaliphatic surface-active reagent. The cured composition obtained has hydrophobicity transfer effect (that is, ability of surfaces to turn hydrophilic pollution into hydrophobic layers) and hydrophobicity recovery effect (that is, ability of surfaces to recover their initial hydrophobic properties after losses resulting, for example, from plasma treatment). However, since the polysiloxane is not chemically built into the network of the cured composition, the OH-terminated polysiloxane and cyclic polysiloxane can be gradually depleted due to migration to the surface, which will result in gradually decreased hydrophobic properties.
Another incumbent approach is synthesizing a siliconized aromatic epoxy interpenetrating polymer network (IPN) by using polyaminoamine as a curing agent and γ-aminopropyltriethoxysilane (γ-APS) as a crosslinking agent for crosslinking diglyceryl ether of bisphenol-A (DGEBA) and OH-terminated polydimethylsiloxane (PDMS). Durability of hydrophobic properties can be imparted to the cured epoxy composition, since PDMS chains are chemically bonded to the network. However, this is an inefficient use of PDMS material because it is evenly distributed throughout the bulk material as opposed to concentrated on the surface. In this case, to increase tracking index of epoxy compositions, large amounts of OH-terminated PDMS are needed, which usually results in decreased tensile and flexural strengths of the cured epoxy composition and is not cost-effective.
Thus, it is desirable to provide an epoxy composition having a surface contact angle with deionized water equal to or greater than 90° (that is, having a hydrophobic surface), resistance to degradation of hydrophobic properties (that is, durable hydrophobic properties), at the same time, without compromising mechanical properties of epoxy compositions. It is also desirable that the hydrophobic surface can be achieved by adding low content of OH-terminated polysiloxane. It is also desirable that an epoxy composition provides hydrophobicity transfer effect and hydrophobicity recovery effect.